A polarizer is a device that produces a polarized light from natural light, wherein the polarized light has a vibration plane in a certain direction and the natural light has a vibration plane in all directions. The polarizer may be divided into an iodine-type polarizer, a dye-type polarizer, a metal-type polarizer, a polyvinylene-type polarizer, an infrared polarizer, etc. Among them, the metal-type polarizer remains to be studied, and the iodine-type polarizing film and the dye-type polarizing film are practically used and commercially available.
The iodine-type polarizing film or the dye-type polarizing film is produced by dyeing a polyvinyl alcohol (PVA)-based film with dichroic iodine, dichroic dye or the like and orienting the dyed PVA-based film in a certain direction using a method such as uniaxial stretching, etc., and has been widely used in LCD industry, etc.
Recently, there have been various attempts to improve an image quality of LCD products as a variety of the LCD products are on the market and there is an increasing demand for the LCD products.
First of all, the white brightness, which controls of brightness of a screen, and the contrast ratio should be high in order to realize a high-definition. The white brightness refers to brightness of the light passed through a polarizing plate, and thus it is difficult to display a high-quality image when the white brightness is low since screen gets dark. Meanwhile, the contrast ratio (CR) represents a brightness ratio of a white state to a black state that is displayed through a pattern generator. Therefore, it is possible to display an image that is close to the original colors as the contrast ratio increases. In this case, the white brightness means brightness of the light emitted through two parallel polarizing plates, and the black brightness means brightness of the light emitted through two polarizing plates in cross. In order to enhance the white brightness, a method for increasing transmission of the light by reducing an amount of a dye that is used to dye a PVA film has been widely used in the art. That is, the black brightness is also increased since the amount of the transmitted light is increased with a decreasing amount of dye absorbed to the PVA film. In this case, a contrast ratio represented by the ratio of the white brightness to the black brightness is decreased since a value of the black brightness is increased in a higher range than a value of the white brightness.
Therefore, it is impossible to realize a clear color image since the brightness of a screen is improved but its contrast ratio is low as the white brightness increases.
Meanwhile, a method for increasing an amount of a dye that is used to dye a PVA film is used in the art so as to improve the contrast ratio, but the method has problems that the entire screen gets dark since the white brightness is reduced by the increased amount of dye.
As described above, the conventional methods have disadvantages that it is difficult to display a high-quality image having high brightness and high contrast ratio since the brightness and the contrast ratio are not improved at the same time.